The present invention relates to the field of sterilization of cryogenic liquids. ln particular, the invention includes a method and apparatus for sterilizing cryogenic liquids by ultrafiltration means.
Cryogenic liquids are used in many industrial and scientific applications. Often it is required that such liquids be extremely pure. One example of an application requiring a very pure cryogenic liquid is the filling of aluminum cans. Because of the relative softness of aluminum, it is impractical to use that material as a container for foods and beverages because the aluminum cans cannot be stacked. The weight of a stack literally crushes the cans at the bottom of the stack. A solution to this problem is to inject a small amount of a cryogenic liquid into the can, as the can is being filled, immediately before the can is sealed. The cryogenic liquid vaporizes rapidly, creating a substantial pressure within the can, which enables the can to withstand the weight of an entire stack. However, because the cryogenic liquid is mixed with the food or beverage, and is ultimatley ingested by the consumer, such liquid must be absolutely free of bacteria, particulates, and other potentially harmful contaminants. Before a cryogenic liquid may be used for this purpose, there must be assurance that the cryogenic liquid is sterile.
Another example of the need for sterilization of cryogenic liquids is in the medical field. Oxygen used by hospitals is often transported and stored as a cryogenic liquid, since storade space requirements are thereby reduced. The liquid is vaporized when gaseous oxygen is needed. Obviously, it is important that the oxygen administered to a patient be sterile. Still another example of an application requiring purified cryogenic liquids is the process of semiconductor chip fabrication. Such processes are carried out at low temperatures, and require states of extreme cleanliness. The cryogenic liquids used in these processes must therefore be free of all unwanted particles. There are several examples, in the prior art, of the use of filters in gas purification. U.S. Pat. No. 2,924,078 discloses a method of making pure liquid oxygen which includes the use of filters and adsorbers. U.S. Pat. No. 3,739,593 discloses a gas separation system which includes the step of filtering a liquefied gas. U.S. Pat. No. 3,653,220 shows a process for purifying helium, which includes the use of molecular sieves to remove impurities by adsorption. U.S. Pat. No. 3,192,730 discloses a method of purifying liquid helium which includes the step of passing superfluid helium through filters.
The examples of the prior art given above relate generally to chemical purification, and not sterilization. Examples of the use of filters having ultrafine mesh sizes, suitable for sterilization purposes, are shown in U.S. Pat. Nos. 3,974,068 and 4,431,545. U.S. Pat. No. 4,150,548 discloses the use of molecular sieves in the removal of pollutants from gas products.
None of the known prior art references discloses a satisfactory method and apparatus for sterilization of a cryogenic liquid by filter means. The present invention provides such a method and apparatus, which is simple in construction. The invention operates on a pure liquid cryogen, and does not require a phase change. The invention can be used with any cryogenic liquid.